1. Field of the Invention
This invention relates to lighting, and particularly to the illumination of areas or beam lighting for use as aircraft landing lights and automobile headlights.
2. Description of the Prior Art
Heretofore, when a lamp, such as a sealed beam incandescent bulb, was used to provide a specific band of electromagnetic radiation, a filter or cold mirror was used to block the unwanted wavelengths. However, the filter or cold mirror trapped heat and the bulb operated at a temperature above the intended operating range. Consequently, the bulb burns out if operated for more than about 15 to 20 minutes and the excessive heat shortens the expected lifetime of the bulb by about 50%. In addition, changing the active spectrum required manual effort or a complex mechanical device to insert or remove the filter.
Another approach uses a dual bulb made with the reflecting surfaces pointing in opposite directions. To change the active spectrum, again a man or a complex mechanical device is needed to rotate the bulb. This approach requires a unique fixture and has relatively high cost.
Another fixture built for use on aircraft consists of four sealed beam lamps, two with cold mirrors and two unfiltered. The fixture, however, is a large complex assembly that is expensive to install on aircraft and also suffers from the bulb overheating problem described above.
Other lights are made using arc lamps, but these are also broad band and require filters with the same difficulties noted above to select specific bands of the spectrum.